The exhaust gas emitted from an internal combustion engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter. Catalyst compositions, typically disposed on catalyst supports or substrates that are disposed within the exhaust system of an internal combustion engine are provided to convert certain or all of these exhaust gas constituents into non-regulated exhaust gas components. For example, exhaust systems for internal combustion engines may include one or more of a precious metal containing oxidation catalyst (“OC”) for the reduction of CO and excess HC, a selective catalyst reduction catalyst (“SCR”) for the reduction of NOx and a particulate filter device (“PF”) for the removal of particulate matter.
An exhaust gas treatment technology in use for high levels of particulate matter reduction, the PF may utilize one of several known exhaust gas filter structures that have displayed effectiveness in removing the particulate matter from the exhaust gas. Such exhaust gas filter structures may include, but are not limited to ceramic honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal fibers, etc. Ceramic wall flow filters have experienced significant acceptance in automotive applications.
The exhaust gas filter is a physical structure for removing particulates from exhaust gas and, as a result, the accumulation of filtered particulates in the exhaust gas filter will have the effect of increasing backpressure in the exhaust system that is experienced by, and that must be overcome by, the internal combustion engine. To address backpressure increases caused by the accumulation of exhaust gas particulates in the exhaust gas filter, the PF is periodically cleaned, or regenerated. Regeneration of a PF in vehicle applications is typically automatic and is controlled by an engine or other suitable controller based on signals generated by engine and exhaust system sensors. The regeneration event involves increasing the temperature of the PF filter, typically by heating the engine exhaust gas, to levels that are often above 600° C. in order to burn the accumulates particulates.
One method of generating the exhaust gas temperatures required in the exhaust system for regeneration of the PF is to deliver unburned HC (ex. Fuel) to an oxidation catalyst device disposed upstream of the PF or to an oxidation catalyst compound disposed in the PF itself. The HC may be delivered to the exhaust system by direct fuel injection into the exhaust system or may be achieved by “over-fueling of” or “late injection of fuel to” the internal combustion engine. The result is unburned HC mixed with the exhaust gas flowing through the exhaust system that is subject to oxidation by the oxidation catalyst in an exothermic reaction that raises the temperature of the exhaust gas. The heated exhaust gas burns the particulate accumulation in the PF. The addition of an oxidation catalyst to the PF can assist in lowering the oxidation temperature of soot and particulates and thus the regeneration temperatures required. This can result in increased durability of the PF and lower HC requirements for regeneration and, therefore, improved fuel economy for the internal combustion engine. In addition, such an oxidation catalyst applied to the PF is useful to oxidize any remaining excess HC in the exhaust gas as well as reducing carbon monoxide constituents (“CO”) resulting from the combustion of soot and particulates.
Increasingly stringent regulations directed to the operation of internal combustion engines, particularly those used in vehicular applications, require monitoring and functional diagnosis of oxidation catalysts disposed in the exhaust gas treatment system including the PF catalyst.